1. Field of the Invention
This invention relates to a semiconductor device having a metal bump formed therein and a method for manufacturing the same, and more particularly to a metal bump type semiconductor device having a metal bump formed directly on an electrode wiring pattern of Al or Al alloy without using a backing metal layer and a method for manufacturing the same.
2. Description of the Related Art
Various wireless bonding techniques for a semiconductor device such as a tape automated bonding (TAB) method, flip-chip method and controlled collapse bonding (CCB) method are known. With these methods, a metal bump formed of Au, solder or the like is generally formed on the electrode of the semiconductor chip and a metal connector or inner lead wire is connected to the metal bump to lead out the electrode of the semiconductor element formed on the chip to the exterior. Pb-Sn (solder) which is inexpensive is frequently used as material of the metal bump.
FIG. 1 shows portion of the prior art semiconductor device including a bump of Pb-Sn. That is, insulation film 2 of SiO.sub.2, for example is formed on silicon substrate 1 and electrode wiring pattern 3 of Al or Al alloy (which is simply referred to as an Al electrode) is formed on insulation film 2. Passivation insulation film 4 formed of SiN film or the like is formed on insulation film 2 and Al electrode 3 to cover the same. Opening 4A is formed in that portion of passivation insulation film 4 which lies on Al electrode 3 so as to expose part of the surface of Al electrode 3. Backing metal layer 5 formed of Cr, Ni, Mo, Cu, Au, Ag or the like is formed on the exposed surface of Al electrode 3. Solder bump 6 is formed on backing metal layer 5 by plating or vapor deposition.
As described above, in the conventional semiconductor device having solder bump 6 formed on Al electrode 3 and a method for manufacturing the same, it is necessary to dispose backing metal layer 5 between Al electrode 3 and solder bump 6 in order to improve the bonding property between Al electrode 3 and solder bump 6. Further, it is necessary to form a mask so as not to plate or vapor deposit solder on any portion other than the electrode portion when solder bump 6 is formed. As a result, the manufacturing process becomes complicated.
In order to solve the above problem, a method of forming a solder bump directly on the Al electrode without using a backing metal plate has been proposed and disclosed in Japanese Patent Disclosure No. 62-104143 made by the same applicant as that of this application. The method disclosed in the above Japanese Patent Disclosure is to form a solder bump by applying ultrasonic waves to molten solder so as to break a natural oxide film formed on the surface of the electrode of Al or Al aloy, for example. This method has received much attention as a wireless bonding method for the semiconductor device.
The principle of the method disclosed in the above Patent Disclosure is as follows: That is, when ultrasonic waves are applied to the molten solder with the molten solder kept in contact with the electrode portion of Al, Al alloy or the like, vaporized metal bubbles occur in the molten solder in the rarefaction or suction phase of the ultrasonic waves and the bubbles will be crushed and disappear in the next compression phase. When the compressed bubbles are destroyed, a strong shock is given to the surface of the Al electrode so that the natural oxide film formed on the Al electrode can be broken and at the same time solder is bonded to the newly created and exposed surface of the Al electrode. In this way, a solder bump is formed directly on the Al electrode without using a backing metal layer.
As a specific method, a wafer is dipped into molten solder in a solder bath and an ultrasonic transducer is inserted into the molten solder to apply ultrasonic waves to the molten solder. Alternatively, ultrasonic waves can be applied to the molten solder by vibrating the solder bath in an ultrasonic mode. Further, ultrasonic waves can be applied to the molten solder while the molten solder is set in contact with the Al electrode by using a soldering iron which can be vibrated in an ultrasonic mode. All the above-described methods are hereinafter referred to as an ultrasonic soldering method. It is generally understood that a solder bump can be formed directly on Al or Al alloy by the ultrasonic soldering method by using solder containing Sn as solder composition.
The above method is simple, but has the following basic problem because Sn-series solder is used and is not yet generally used in the field of a semiconductor industry, for example. The first problem is related to formation of an Al-Sn alloy layer and the second problem is related to absorption phenomenon of the base metal of Al or Al alloy caused by the presence of molten solder. The problems are explained in detail.
First, the first problem is explained. For example, in a case where a Pb-Sn eutectic alloy is used as the Sn-series solder and a solder bump is formed directly on an Al electrode, an Al-Sn alloy layer is formed between the Al electrode and the solder bump. A potential difference occurs between the Al-Sn alloy layer and the Al electrode to form a parasitic battery cell (or eutectic battery cell), thereby corroding the Al electrode. The Al-Sn alloy layer further grows when held at high temperatures (higher than 100.degree. C.), further corroding the Al electrode and significantly degrading the bonding property of the junction between the solder and Al. In particular, when the solder bump is melted so as to be connected to a metal connector or inner lead for electrode lead-out, for example, after the solder bump is formed, the above alloy layer grows larger irrespective of the Pb-Sn composition ratio of the solder, significantly degrading the bonding property of the junction between the solder and Al.
Next, the second problem is explained. When the molten solder is set in contact with Al, a so-called absorption phenomenon in which Al is melted or absorbed into the solder occurs and proceeds at a relatively high speed irrespective of the Pb-Sn composition ratio. Since the processing time for forming the solder bump by the ultrasonic soldering method can be made shorter than several seconds, influence by the absorption action of Al can be suppressed so as to be negligible. However, when the solder is meted again to connect the bump to an inner lead or the like in the later step and if it is held at high temperatures for more than several tens of seconds, the absorption action of Al may proceed significantly. As a result, a portion of Al except the electrode portion on which the bump is formed is eaten by or absorbed into the solder, making it extremely difficult to selectively and precisely form the bump. Further, a top passivation film formed of SiN, for example, for chip protection is generally formed on the top layer of the semiconductor chip. If the SiN film is formed on the Al electrode, the solder bump can be formed with the SiN film used as a block film. At this time, if the absorption action proceeds and the solder penetrates into portion of the Al electrode under the SiN film, the SiN film may crack and cannot have a protection function as a passivation film.
As described above, in the conventional technique in which the solder bump is formed on the electrode wiring pattern containing Al as a main part through the backing metal layer, the manufacturing process becomes complicated and the productivity is lowered. It is possible to form the solder bump directly on the Al electrode by use of the ultrasonic soldering method using Sn-series solder. However, in this case, an Al-Sn alloy layer is formed and the absorption phenomenon of Al will occur. In particular, if the structure is placed under a high temperature condition in the step of connecting a metal connector or inner lead after the bump is formed, the Al-Sn alloy layer further grows and the absorption action further proceeds. As a result, the reliability of the semiconductor device is degraded.
In order to solve the above problems, the inventors made various experiments by ultrasonic soldering methods using solder containing Ag in addition to Pb-Sn. The presence of Ag caused Ag .sub.3 Al (Ag .sub.2 Al) to be formed and prevented formation of an Al-Sn alloy layer. However, it was confirmed that the Ag-Al alloy layer was weak, and like the Al-Sn alloy layer, the weak Ag-Sn alloy layer will grow when the solder bump is melted again, thus making it impossible to attain a sufficiently high bonding strength. That is, the above problems could not be solved.